I'm with Bernie. When I was flying 1/2A Combat (seriously over-competitive, going to the Nats and many contests each year) I ran the black Cox 5X3 (cut to 4.5 X 3) more than any other prop.

Sometimes we ran glass or carbon props, but usually the Cox.

We broke a few, but more often we just finally wore them out. They are soft rubber (plastic?) and if you run them into the ground enough times they will finally get too soft to hold the proper pitch.

I my mind it is an easy selection to go with the Black (or yellow) Cox in 6X3 or 5X3 for .049s.